1. Field of the Invention
The present invention relates to a power inverter that converts Direct Current (DC) power into Alternate Current (AC) power and vice versa.
2. Description of Related Art
The power inverter includes a function with which DC power supplied from a DC power supply is converted into AC power to be supplied to an AC electricity load such as an electric rotating machine, and a function with which AC power generated by an electric rotating machine is converted into DC power to be supplied to a DC power supply. To perform the conversion function, the power inverter further includes an inverter circuit which includes a switching device, and by repeating conduction operation and interrupt operation with the switching device, the power inverter converts from DC power to AC power or AC power to DC power.
Spike voltage is generated by inductance that exists in the circuit because the switching operation cuts off the current. For reducing the spike voltage, it is preferable to provide a smoothing capacitor and to reduce inductance at a DC circuit. A technology for controlling spike voltage by reducing inductance is disclosed in Japanese Laid Open Patent Publication No. 2002-34268 (patent document 1). According to the patent document 1, inductance is reduced by shortening wiring between the smoothing capacitor and the switching device so as to reduce the spike voltage.
An in-car power inverter is provided with DC power from an in-car battery and converts the DC power into three-phase AC power to be supplied to an electric rotating machine for vehicle, etc. As demand for torque generated by an electric rotating machine for vehicle is larger than that of early days, the power converted by a power inverter tends to be larger. The in-car power inverter is used under high-temperature environment in comparison to a power inverter for general industrial machinery installed in factories. It is hence desirable for the in-car power inverter to reduce heat generated by the power inverter itself in comparison to the general power inverter. Most of the heat generated by the power inverter itself is the heat generated by the switching device that constitutes the inverter circuit. It is thus desirable to reduce the heat generated by the switching device as much as possible.
Heating value by the switching device increases at switching from the interrupting state to the conducting state and at switching from the conducting state to the interrupting state. Therefore, it is desirable to reduce the heat at the switching operation. The first countermeasure for reducing the heating value is to shorten switching operation time of the switching device. The second countermeasure is to lengthen intervals between the switching operations for the switching device, in other words, to reduce the number of switching device operations per unit time. However, to extremely lengthen intervals between the switching operations for the switching device may reduce the accuracy of control. To greatly reduce the number of switching device operations per unit time has limit.
Japanese Laid Open Patent Publication No. 2007-143272 (patent document 2) discloses a technology for shortening switching operation time of a switching device of an inverter circuit to reduce heating value per switching operation of the switching device by lowering inductance.
As described above, the technology disclosed in the patent document 1 does not concern about reducing the heat at the switching operation of the switching device.
The patent document 2 discloses the technology for realizing low inductance that results in reducing the heat value per switching operation of switching device. The patent document 2, however, has a challenge in reducing the heat value and reducing the size of the power inverter because the power inverter, particularly the in-car power inverter, does not have enough space around it.
As electric energy converted by the power inverter increases, equipments tend to get bigger. So, it is preferable to minimize the volume of the equipments regardless of increase in the electric energy, in other words, for example, to increase the value of maximum convertible electric power per unit volume of the power inverter. For that, it is desirable to achieve a balance between low inductance and reduction in the size. Here, reduction in the size refers to maximization of the value of maximum convertible electric power per unit volume of the power inverter.